On 2006-08-02, jasen wrote:

I recently tested a 9V alkaline battery by measuring its open-circuit
voltage (9.0 V) and then measuring it with a car headlight lamp (R = 1
Ohm) across the terminals (4.0 V). The lamp lit up brightly and got
warm, but from the significant voltage drop I conclude that the
battery is basically dead. Correct?
....
If you've got lots of spare automobile parts there,
perhaps "3W" panel indicator lamp would be more realistic
or a 22 ohm resistor.

Perhaps I should mention that the only reason I did this test was that
I happened to notice a used 9V battery and a headlamp lying near each
other on my workbench, and curiosity drew me in!


Right. I measured the lamp's resistance with an ohmmeter, which of
course puts very little current through it.

But I took the measurements by clipping the voltmeter (actually it's
the same meter) leads onto the battery terminals, reading the
open-circuit voltage, then pressing the lamp's terminals against the
battery terminals (the spacing was convenient --- that's where I got
the idea from) and immediately reading the loaded voltage (before the
lamp heated up).

I wrote those two paragraphs in the wrong order. I measured the
voltage drop first and then (after letting the lamp cool) measured its
resistance.

If I'd seen the 1 Ohm value first, I wouldn't have used it for the
voltage-drop test!